Digital Radiography (DR) systems offer benefits and advantages over film-based systems and are used increasingly for diagnostic imaging in numerous applications. Unlike its earlier film predecessor, the DR detector panel forms an image by collecting electronic signals from thousands of pixels in its detector array, each signal level indicative of the amount of X-ray radiation that is detected. While this arrangement can serve as a capable alternative to the conventional film cassette and offers a number of capabilities for improved image processing and display, however, there can be considerable complexity involved in calibrating the DR panel so that it delivers a uniform and well-characterized response over the full range of radiation levels received.
A related area of difficulty that is familiar to those skilled in the diagnostic imaging arts relates to differences in response to exposure from one imaging system to the next, particularly between the response of earlier film-based systems and the response of systems using DR digital sensors. In general, film response follows the familiar D-log E sigmoid-shaped curve and allows a more limited range than does digital response. Digital sensors offer a wider dynamic range for clinical imaging, in part, because their output data can be readily manipulated to compensate for response characteristics. Differences such as these between digital and film detection characteristics for X-ray exposure have been noted and addressed in the art using various techniques, adapted for obtaining clinically useful images.
The response of the DR detector is calibrated before a system can be used to acquire clinical images. This detector calibration serves a number of purposes, including: (i) compensating for differences among individual sensors that relate to sensitivity and to dark signal response; (ii) compensating for differences in gain and offset response among individual signal channels of the amplification and digitization electronics; (iii) compensating for non-uniformity of the X-ray field; (iv) establishing a known (or relative) arithmetic relationship (for example, linear or logarithmic) between detector response (for example, analog-digital converter or ADC counts) and incident radiation dosage at the detector surface; and (v) additional processing, for example, identifying defective pixels, could also be part of the detector calibration task.
As described earlier, the response of digital devices is generally linear, at least to an approximation of the first order. Thus, in conventional practice, DR systems typically use a simple linear correction to calibrate for detector response. However, it is known that linear extrapolation may be inaccurate over portions of the exposure range. The graph of FIG. 1 shows exemplary detector response to exposure for various DR detectors. A dotted line 42 indicates ideal linear response for a DR detector. The actual response of a detection component is more accurately shown, in this example, by a solid curve 40. As can be seen from this graph, the response of the DR detector is substantially linear for input exposures of up to about 6 mR in this example. As exposure levels increase above this level, however, a simple linear calibration becomes progressively less accurate. If a linear approximation were used where there is such a response, image artifacts would appear over higher exposure regions after the linear correction is applied. For many types of images, this could result in banding, with strips appearing near a skin line for a conventional chest X-ray image. Non-linear response for higher exposure values applies, for example, for the Trixell Pixium 4600 detector from Trixell Inc., Morains, FR, and other similar devices.
It is noted that the problem exhibited in FIG. 1 can be more or less noticeable at different kVp levels. Thus, for example, the same DR detector may exhibit different response characteristic and a different amount of non-linearity depending on the particular exposure kVp settings that are used.
If the response of a DR detector were truly linear, it would only be necessary to measure response at two different arbitrary exposure levels, including zero exposure level (i.e. dark) where no radiation is given. Because any two points along the line are sufficient to define the line, this would provide sufficient offset and gain information for linear calibration. However, where linear response does not apply, as shown in the upper range of the example of FIG. 1, the response of the detector needs to be measured at more than two exposure points in order to characterize its response behavior more accurately. This would enable its response to be more suitably characterized with a higher order polynomial formula or other appropriate non-linear mathematical approximation function. Moreover, for non-linear mapping, it now becomes relevant to identify, as closely as possible, the input exposure levels (or, at least, relative relationship between the exposure levels) at which the detector responses are to be measured. Doing this allows proper correction to be applied for a raw image with improved accuracy, resulting in eliminating or at least diminishing image artifacts such as banding.
Conventionally, determining the exposure levels used in DR detector calibration requires use of dosimetric instruments and involves tedious routines with repetitive setup, exposure, and reading and processing operations. For example, once the X-ray exposure technique is chosen, the actual exposure reaching the detector is typically measured with a dose meter placed in front of the detector. Additional measurements are required if more exposure levels are used. The exposure information that is obtained is not only subject to error due to accuracies of the dosimetric instrument and the experimental setup, but also depends on the stability of the radiation output of the X-ray generator unit. In addition, when a detector calibration needs to be repeated at a later time, the exposure levels measured from previous calibration setup would need to be re-measured. This is because any variations in the experimental setup, including geometry differences and X-ray generator output variation, for example, can change the radiation exposure reaching the detectors and thus compromise the accuracy of the calibration result.
Clearly, performing thorough DR detector calibration involving multiple exposure levels can become a tedious and labor-intensive process. As a result, compromises such as linear approximation and interpolation are often made in practice, in order to save cost and labor, to minimize error, and to minimize overall calibration downtime.
With the growing importance of DR diagnostic imaging and the drive to improve diagnostic tools and the efficiency of medical care in general, there is, then, a need for improved methods for DR detector calibration that can be performed at reduced cost, that allows a high degree of accuracy, and that offer the capability for automation without extensive operator activity or interaction.